This invention relates to shrink films made from pseudohomogeneous linear low density polyethylene.
The term xe2x80x9cshrink filmxe2x80x9d refers to a plastic wrapping film which has the characteristic of shrinking when it is heated to near the melting point of the film. These films are commonly manufactured from plastic resins such as polyvinyl chloride (PVC); polypropylene (PP); linear-low density polyethylene (LLDPE); low density polyethylene (LDPE); high density polyethylene (HDPE); copolymers of ethylene and vinyl acetate (EVA); copolymers of ethylene and vinyl alcohols (EVOH); ionomers (e.g. Surlyn(trademark)); copolymers of vinylidene chloride (e.g. PVDC, SARAN(trademark)); copolymers of ethylene acrylic acid (EAA); polyamides (PA); among others.
End uses of these films include food packaging (for example, oxygen and moisture barrier bag for frozen poultry, primal meat cuts and processed meat and cheese products for preservation of freshness and hygienics) and non-food packaging (for example, xe2x80x9coverwrapsxe2x80x9d for protecting goods against damage, soiling, tampering and pilferage) during transportation, distribution, handling and display. An example of end use is found in retail sales where the films are wrapped air-tight around single or multiple items of compact disks, audio/video tapes, computer software boxes, magazines, confectionery, boxed products, single serve bowls, etc. Another example of end use is found in wholesale retailing where multiple containers of bottled and canned goods such as beverages, condiments and personal hygiene products are sold in bulk. Yet another example is found in courier shipping where singular items of shrink-wrapped sporting goods and household appliances are now safely transported without the need for bulky protective cardboard cartons.
Films of the present invention must be made from linear low density polyethylene. Linear low density polyethylene (LLDPE) is conventionally prepared by a polymerization process using a so-called Ziegler Natta catalyst. It is well known to those skilled in the art that the conventional LLDPE resins prepared with Ziegler Natta catalysts do not have a uniform structure or composition. In particular, these conventional resins typically contain a minor amount of a very low density copolymer; a major portion of the xe2x80x9ccopolymerxe2x80x9d having the desired molecular weight and density; and a large xe2x80x9chomopolymerxe2x80x9d fraction (which does not contain a meaningful amount of the desired comonomer). This lack of polymer homogeneity is associated with several disadvantages; for example 1) xe2x80x9corganolepticxe2x80x9d problems caused by the low molecular weight material; and 2) suboptimal impact strengths which are believed to be caused by the crystallinity of the homopolymer fraction. The development of xe2x80x9chomogeneous polyethylenexe2x80x9d resins has mitigated these disadvantages.
Homogeneous resins may be prepared with the so-called metallocene catalysts which are well known and widely described in the literature.
The resulting xe2x80x9chomogeneousxe2x80x9d resins have a very uniform composition as evidenced by the substantial absence of very low molecular weight/low density fractions and/or homopolymer fractions. These homogeneous resins exhibit excellent organoleptic properties and impact strength properties. However, films made with homogeneous resins often have deficient tear properties. In addition, shrink properties may become xe2x80x9cunbalancedxe2x80x9d (with, for example, a greater shrink force in the machine direction then the transverse direction (TD) or even a negative TD shrinkage) when shrink films are made with highly homogeneous resins.
U.S. Pat. Nos. 5,591,390 and 5,595,705 (the disclosures of which are incorporated herein by reference) describe the preparation of shrink films made with a sub group of homogeneous resins, namely homogeneous resins having long chain branching. However, such resins are comparatively expensive.
The present invention provides a shrink film which mitigates certain of the disadvantages of films made from either xe2x80x9cconventionalxe2x80x9d and xe2x80x9chomogeneousxe2x80x9d polyethylene resins.
The present invention provides a monolayer shrink film having, in non-crosslinked form, 1) a dart impact strength as determined by ASTM D1709 of greater than 450 grams per mil; 2) a tear strength as determined by ASTM D1922 of greater than 25 grams per mil in the transverse direction; and 3) a shrink tension of greater than 400 pounds per square inch (psi) in the machine direction; wherein said shrink film is prepared from a pseudohomogeneous linear low density polyethylene having a COHO ratio of from 3.5/1 to 19/1.
While not wishing to be bound by any theory, it is believed that the excellent properties of the films of this invention are attributable to two essential characteristics of the LLDPE used to make them, namely a combination of (a) a comparatively broad molecular weight distribution and (b) the presence of a pseudohomogeneous comonomer distribution (as evidenced by the copolymer/homopolymer or COHO ratio).
It is believed that the xe2x80x9cpseudohomogeneousxe2x80x9d LLDPE resins of the type described above have not been heretofore commercially available. We have discovered that shrink film made from these resins has a very surprising and highly desirable balance of strength properties and shrink properties which are particularly well suited for mono- or multilayer film constructions for food or non-food packaging.
Shrink films must have excellent strength properties and puncture resistance in addition to the obvious need for shrink characteristics to conform to the shape of the packaged goods and therefore allow for air-tightness and attractive shelf appearance.
It is well known to xe2x80x9ccrosslinkxe2x80x9d the films (for example, by irradiation) and/or to prepare multilayer shrink films in order to improve these xe2x80x9cstrengthxe2x80x9d and xe2x80x9cpuncturexe2x80x9d properties as well as to improve barrier properties. The films of the present invention have excellent strength and puncture properties in the form of an uncrosslinked, monolayer film. However, it is also with the scope of the invention to further improve the films of the present invention via crosslinking and/or xe2x80x9cmulti-layeringxe2x80x9d technique.
Multilayer films are most commonly prepared by coextrusion (as opposed to lamination). Such films are available in constructions of from two to eleven layers. Films having between three and nine layers are typical at the present time. The use of the film of the present invention as the xe2x80x9ccore layer(s)xe2x80x9d (i.e. one or more of the non-surface layers) of a multilayer construction is particularly preferred.
It is also within the scope of the present invention to prepare a multilayer film having at least one layer of film according to the present invention and one or more layers prepared from completely different plastic resins (such as LDPE, PP or EVA).
Shrink film is prepared by a blown film extrusion process (as opposed to a xe2x80x9ccastxe2x80x9d film process). The blown film process is well known to those skilled in the art.
Shrink film is now typically produced using two different processes known to those skilled in the art as 1) the xe2x80x9ctenter framingxe2x80x9d or 2) the xe2x80x9cdouble bubblexe2x80x9d process. In the double bubble process, an initial tube is extruded and then cooled or quenched (often using a water cooling system) before being subsequently expanded (and oriented in both the machine and transverse direction) through a heating oven.
The preparation of a multilayer shrink film is described in more detail in U.S. Pat. No. 4,551,380 (Shoenberg) and the preparation of a monolayer shrink film is described in U.S. Pat. No. 4,760,116 (Roberts). The detailed descriptions which are provided in these patents, and the aforementioned U.S. Pat. Nos. 5,591,390 and 5,595,705, are incorporated herein by reference.
As previously noted, the films of the present invention must be prepared using a pseudohomogeneous linear low density polyethylene resin. As used herein, the term xe2x80x9cpseudohomogeneousxe2x80x9d means that the resin has a copolymer/homopolymer (or xe2x80x9cCOHOxe2x80x9d) ratio of from 3.5/1 to 19/1 and is preferably from 4.0 to 6.0. Stated alternatively, this means that the pseudohomogeneous resins of the present invention are from about 78% copolymer (corresponding to a COHO ratio of 3.5/1) to 95% copolymer (corresponding to a COHO ratio of 19/1) with the balance being homopolymer. Similarly, the preferred copolymers are from 80% copolymer to about 86% copolymer with the balance being homopolymer. In contrast, conventional LLPDE resins of the same density (made for example, with conventional Ziegler Natta catalysts in conventional polymerization reactors) generally have a COHO ratio of less than 4 (with a COHO ratio of from 2 to 3 being common) and xe2x80x9chomogeneousxe2x80x9d resins (i.e. resins made with a so-called single site catalyst, such as a metallocene catalyst) are effectively 100% copolymer (i.e. with no appreciable amounts of homopolymer being detectable by conventional analytical techniques).
The LLDPE resins used in the present invention are preferably prepared in using a highly efficient Ziegler Natta catalyst and in a very well mixed solution polymerization reactor. In another preferred embodiment of this invention, a xe2x80x9cdual reactorxe2x80x9d polymerization process is used to broaden the molecular weight distribution (xe2x80x9cMWDxe2x80x9d) of the preferred LLDPE.
A. Description of Ziegler Natta Catalyst
The term xe2x80x9cZiegler Natta catalystxe2x80x9d is well known to those skilled in the art. A Ziegler Natta catalyst may be used in this invention. Ziegler Natta catalysts comprise at least one transition metal compound of a transition metal selected from groups 3, 4 or 5 of the Periodic Table (using IUPAC nomenclature) and an organoaluminum cocatalyst which is defined by the formula:
Al(Xxe2x80x2)a(OR)b(R)c
wherein: Xxe2x80x2 is a halide (preferably chlorine); OR is an alkoxy or aryloxy group; R is a hydrocarbyl (preferably an alkyl having from 1 to 10 carbon atoms); and a, b or c are each 0, 1, 2 or 3 with the provisos text a+b+c=3 and b+cxe2x89xa71.
It is preferred that the transition metal compounds contain at least one of titanium or vanadium. Exemplary titanium compounds include titanium halides (especially titanium chlorides, of which TiCl4 is preferred); titanium alkyls; titanium alkoxides (which may be prepared by reacting a titanium alkyl with an alcohol) and xe2x80x9cmixed ligandxe2x80x9d compounds (i.e. compounds which contain more than one of the above described halide, alkyl and alkoxide ligands). Exemplary vanadium compounds may also contain halide, alkyl or alkoxide ligands. In addition vanadium oxy trichloride (xe2x80x9cVOCl3xe2x80x9d) is known as a Ziegler Natta catalyst component and is suitable for use in the present invention.
As will be appreciated by those skilled in the art of ethylene polymerization, conventional Ziegler Natta catalysts may also incorporate additional components such as an electron donor (for example an amine or an ether) and/or a magnesium compound (for example a magnesium alkyl such as a butyl ethyl magnesium). A halide source (which is typically a chloride such as tertiary butyl chloride) is typically used when a magnesium compound is present.
Such components, if employed, may be added to the other catalyst components prior to introduction to the reactor or may be directly added to the reactor.
It is highly preferred that the Ziegler Natta catalyst contains a titanium compound, a magnesium alkyl compound and a chloride compound and that an aluminum alkoxide is used as the cocatalyst.
The Ziegler Natta catalyst may also be xe2x80x9ctemperedxe2x80x9d (i.e. heat treated) prior to being introduced to the reactor (again, using techniques which are well known to those skilled in the art and published in the literature). Particularly preferred Ziegler Natta catalysts and methods of preparing them are described in U.S. Pat. Nos. 5,492,876; 5,519,098; and 5,589,555.
B. Description of Solution Polymerization Process
Solution processes for the copolymerization of ethylene and an alpha olefin having from 3 to 12 carbon atoms are well known in the art. These processes are conducted in the presence of an inert hydrocarbon solvent typically a C5-12 hydrocarbon which may be unsubstituted or substituted by a C1-4 alkyl group, such as pentane, methyl pentane, hexane, heptane, octane, cyclohexane, methylcyclohexane and hydrogenated naphtha. An example of a suitable solvent which is commercially available is xe2x80x9cIsopar Exe2x80x9d (C8-12 aliphatic solvent, Exxon Chemical Co.).
The solution polymerization process of this invention preferably uses at least two polymerization reactors. The polymer solution exiting from the first reactor is preferably transferred to the second polymerization (i.e. the reactors are most preferably arranged xe2x80x9cin seriesxe2x80x9d so that polymerization in the second reactor occurs in the presence of the polymer solution from the first reactor).
The polymerization temperature in the first reactor is from about 80xc2x0 C. to about 180xc2x0 C. (preferably from about 120xc2x0 C. to 160xc2x0 C.) and the second reactor is preferably operated at a slightly higher temperature. Cold feed (i.e. chilled solvent and/or monomer) may be added to both reactors or to the first reactor only. The polymerization enthalpy heats the reactor. The polymerization solution which exits the reactor may be more than 100xc2x0 C. hotter than the reactor feed temperature. The polymerization reactor(s) must be xe2x80x9cstirred reactorsxe2x80x9d (i.e. the reactors are extremely well mixed with a good agitation system). Agitation efficiency may be determined by measuring the reactor temperature at several different points. The largest temperature difference (i.e. between the hottest and coldest temperature measurements) is described as the internal temperature gradient for the polymerization reactor. A very well mixed polymerization reactor has a maximum internal temperature gradient of less than 10xc2x0 C. A particularly preferred agitator system is described in copending and commonly assigned U.S. Pat. No. 6,024,483. Preferred pressures are from about 500 psi to 8,000 psi. The most preferred reaction process is a xe2x80x9cmedium pressure processxe2x80x9d, which means that the pressure in each reactor is preferably less than about 6,000 psi (about 42,000 kilopascals or kPa), and most preferably from about 1,500 psi to 3,000 psi (about 14,000-22,000 kPa).
Suitable monomers for copolymerization with ethylene include C3-12 alpha olefins which are unsubstituted or substituted by up to two C1-6 alkyl radicals. Illustrative non-limiting examples of such alpha-olefins are one or more of propylene, 1-butene, 1-pentene, 1-hexene, 1-octene and 1decene. Octene-1 is highly preferred.
The monomers are dissolved/dispersed in the solvent either prior to being fed to the first reactor (or for gaseous monomers the monomer may be fed to the reactor so that it will dissolve in the reaction mixture). Prior to mixing, the solvent and monomers are generally purified to remove potential catalyst poisons such as water, oxygen or other polar impurities. The feedstock purification follows standard practices in the art, e.g. molecular sieves, alumina beds and oxygen removal catalysts are used for the purification of monomers. The solvent itself as well (e.g. methyl pentane, cyclohexane, hexane or toluene) is preferably treated in a similar manner. The feedstock may be heated or cooled prior to feeding to the first reactor. Additional monomers and solvent may be added to the second reactor, and it may be heated or cooled.
Generally, the catalyst components may be premixed in the solvent for the reaction or fed as separate streams to each reactor. In some instances premixing may be desirable to provide a reaction time for the catalyst components prior to entering the reaction. Such an xe2x80x9cin line mixingxe2x80x9d technique is described the patent literature (most notably U.S. Pat. No. 5,589,555, issued Dec. 31, 1996 to DuPont Canada Inc.).
The residence time in each reactor will depend on the design and the capacity of the reactor. Generally the reactors should be operated under conditions to achieve a thorough mixing of the reactants. In addition, it is preferred (for dual reactor operations) that from 20 to 60 weight % of the final polymer is polymerized in the first reactor, with the balance being polymerized in the second reactor. As previously noted, the polymerization reactors are preferably arranged in series (i.e. with the solution from the first reactor being transferred to the second reactor). In a highly preferred embodiment, the first polymerization reactor has a smaller volume than the second polymerization reactor. On leaving the reactor system the solvent is removed and the resulting polymer is finished in a conventional manner.
Further details of the invention are illustrated in the following, non-limiting, examples. The examples are divided into three parts.
The first part illustrates the copolymerization of ethylene and octene-1 in a dual polymerization reactor system using a Ziegler Natta catalyst.
The second part illustrates the preparation of the inventive films in a double bubble film extrusion process.
The third part illustrates the actual use of the inventive films in a shrink packaging operation.
1. Melt Index: xe2x80x9cI2xe2x80x9d, xe2x80x9cI6xe2x80x9d, xe2x80x9cI21xe2x80x9d and Melt Flow Ratio (which is calculated by dividing I21 by I2) were determined according to ASTM D1238. [Note: I2 measurements are made with a 2.16 kg weight and I21 measurements are made with a 21.6 kg weight at 190xc2x0 C.] Test results are reported in units of grams/10 minutes (though these units are often omitted by convention).
2. Stress Exponent (xe2x80x9cS.Exxe2x80x9d) is calculated by             log      ⁡              (                              I            6                    /                      I            2                          )                    log      ⁡              (        3        )              .
[Note: Stress Exponent may be regarded as a proxy for molecular weight distribution (i.e. an increase in Stress Exponent value suggests a broadening of molecular weight distribution).]
3. Number average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity (calculated by Mw/Mn) were determined by high temperature Gel Permeation Chromatography xe2x80x9cGPCxe2x80x9d with differential refractive index xe2x80x9cDRIxe2x80x9d detection using universal calibration.
4. 1% Secant Modulus (MD/TD) was determined according to ASTM D882.
5. Elongation and Yield measurements were determined according to ASTM D882.
6. Melt strength is determined using the same xe2x80x9cmelt indexerxe2x80x9d apparatus used in the aforementioned ASTM D1238 test method. The apparatus is loaded with resin and preheated for 6 minutes to 190xc2x0xc2x10.20xc2x0 C. as per ASTM D1238. The total piston load used is 18,400 g (consisting of the piston weight of 100 g and a 18,300 g weight). After heating, the polymer is allowed to extrude from the melt indexer die until the piston travels to a predetermined starting point and the strand is removed by cutting. The piston continues to its end point and stops moving. A timing device (e.g. a stopwatch) is activated. The timer is stopped at the instant the extrudate thread falls off the die orifice. The amount of extrudate is then weighed. This procedure is repeated 3 to 5 times yielding different extrudate weights and corresponding times. The results are then plotted on a log-log scale (weight of extrudate in grams versus time in minutes). xe2x80x9cMelt strengthxe2x80x9d is reported as the value (in grams) at the three minute time as is expressed, for example as 0.15 g/3 min.
7. Tensile measurements were made according to ASTM D882.
8. Tear measurements were made according to ASTM D1922.
9. Density was determined using the displacement method according to ASTM D792.
10. Copolymer/homopolymer (xe2x80x9cCOHOxe2x80x9d) determinations were made using the Temperature Rising Elution Fractionation (or xe2x80x9cTREFxe2x80x9d) technique which is well known to those skilled in the art and widely described in the literature. As will be understood by those skilled in the art, the xe2x80x9ccopolymerxe2x80x9d and xe2x80x9chomopolymerxe2x80x9d fractions of the resin have substantially different branching contents and therefore the corresponding crystallization and dissolution temperatures. The results from this analysis are conventionally expressed as a copolymer/homopolymer (or xe2x80x9cCOHOxe2x80x9d) ratio. A COHO ratio of 4 indicates that the resin has four parts by weight of copolymer per part by weight homopolymer (or 80% copolymer). Likewise, a COHO ratio of 19 corresponds to 95% copolymer.
11. Shrink force and shrink tension were determined by ASTM D-2838 using an instrumented shrink tension tester made by Dek-Tron Scientific Instruments of Plainfield, N.J.
12. Free shrink (expressed as % shrinkage of total initial area) was determined on specimens having dimensions of 4.0 inch by 4.0 inch (about 10 cm by 10 cm) according to ASTM D-1204 for a 10 minute duration at 110xc2x0 C.
13. Branch content distribution was determined by TREF-GPC (Temperature Rising Elution Fractionationxe2x80x94Gel Permeation Chromatography). TREF-GPC combines two techniques well known to those skilled in the art of polyethylene characterization and the procedure is briefly described here. The polymer sample is dissolved in solution and crystallized under dilute condition. Polymer was then separated and collected into different fractions according to solubility which is dependent on comonomer content. The comonomer content (as determined by FTIR), the molecular weight distribution (as determined by GPC) and the amount of polymer in each fraction were then determined. Mathematical summation of the fractions provides a distribution of molecular weight and comonomer content for the entire sample.
14. Puncture strength measurement was determined as follows. An instrumented physical properties testing machine (Instron 4204 Universal Testing Machine) equipped with 1 kilo Newton load cell and a 1xc2xdxe2x80x3 (38.1 mm) tapered probe head coated with poly(tetrafluoroethylene) (sold under the trademark TEFLON(copyright)) are used. A film sample is clamped beneath the probe head with the probe head and the film liberally coated with a water soluble lubricant. The probe head is then pushed through the sample at a speed of 20xe2x80x3/min (500 mm/min) to the rupture point. The final result is calculated based on the energy at failure normalized to 1 mm film thickness for a minimum of five replicate analyses. The test results are shown in Tables 3 and 4 in the row entitled xe2x80x9cSlow Puncture with Teflonxe2x80x9d and are expressed in Joules per millimeter.